The present invention relates generally to sound detectors and more particularly to optical systems which detect acoustic signals.
A new class of sound detectors which detect acoustically-induced phase-shifts in light beams traveling through optical fibers has been described in U.S. Pat. No. 4,162,397 to J. A. Bucaro et al., issued July 24, 1979 and in U.S. Pat. No. 4,297,887 issued Nov. 3, 1981 to J. A. Bucaro, both assigned to the same assignee as the present case. In the sound detector disclosed in U.S. Pat. No. 4,162,397, a laser beam is split, one part of the beam being sent into a sensing optical fiber and another part of the beam being sent into a reference optical fiber. The sensing optical fiber is placed in a fluid medium in which an acoustic pressure wave signal is to be detected, while the reference fiber is kept at a remote location where it can be isolated from the acoustic pressure wave signal. The acoustic pressure signal alters the optical path length of the sensing fiber through strain-induced index of refraction changes and pressure-induced fiber length changes. Added sensitivity can be provided by application of a suitable elastomeric or plastic coating to the fiber. The coating elongates when subjected to pressure, thereby stretching the inner glass fiber. The optical-path-length changes induced in the sensing fiber by the acoustic pressure wave signal cause the phase of the beam passing through the sensing fiber to be modulated by the waveform of the acoustic pressure wave signal. In homodyne operation of the detection system, the two parts of the split laser beam are recombined and mixed on the surface of a photodetector to produce an electrical signal having a zero frequency component that carries the phase modulation as an equivalent frequency modulation. The zero-frequency electrical signal component is demodulated and the waveform of the acoustic pressure wave signal is recovered.
In order to eliminate low frequency 1/f noise from the photodetector, the detection system is preferably operated in a heterodyne mode, rather than the homodyne mode described above. In the heterodyne mode of operation, an optical modulator such as a Bragg cell is introduced in the reference fiber to shift the frequency of the part of the split laser beam passing through the reference fiber. When the two parts of the split laser beam are recombined and mixed on the surface of the photodetector the component of the electrical signal carrying the phase modulation has a non-zero frequency which is equal to the difference of the frequencies of the two parts of the split laser beam passing through the fibers. As before, the difference frequency component is demodulated and the waveform of the acoustic pressure wave signal is recovered. However, since the modulated component of the electric signal is now a high frequency component, low frequency 1/f noise is eliminated.
The disclosed sound detector suffers from several disadvantages. Firstly, the reference fiber must be physically separated from the sensing fiber, since if the acoustic pressure wave alters the optical path length of both fibers, there is no net effect. Secondly, since the two fibers are physically separated, they will experience different temperature fluctuations. As little as millidegree temperature fluctuations in the two fibers results in extremely large noise signals generated in the photodetector current. Thirdly, sound waves acting along the fiber leading to the pair of reference and sensing fibers are also detected. This leads to problems when level sound detection is desired or in remote sensing with very long lead fibers. Finally, the coherence length of the laser must be at least as long as the total optical path length difference between a beam traveling through the sensing fiber and one traveling through the reference fiber. If low tolerence sources are utilized, the path lengths of the fibers must be carefully matched.
The sound detector disclosed in U.S. Pat. No. 4,297,887 seeks to eliminate these problems. Essentially, this sound detector involves using a reference fiber whose acoustic sensitivity is different from that of the sensing fiber and coiling the fibers together as a pair. In this approach, both the reference fiber and the associated optical modulator are placed, along with the sensing fiber, in the fluid medium rather than being kept at the remote location. This requires that electrical power and signals for the optical modulator be sent all the way to the point where the acoustic pressure wave signals are to be detected. However, the full potential of the sound detector is realized when only optical signals need be conveyed to and from the fluid medium in which the acoustic pressure wave signals are to be detected.